760
Macromolecules 1987,20, 760-767
Vinylpyridinium Ionomers. 2. Styrene-Based ABA Block Copolymers Sylvie Gauthiert and Adi Eisenberg* Department of Chemistry, McGill University, Montreal, Quebec, Canada H3A 2K6. Received July 23, 1986
ABSTRACT The thermal and dynamic mechanical behavior of styrene-4-vinylpyridinium ABA block ionomers associated with the was investigated as a function of ion content and method of preparation. Only one Tg, glass transition of the polystyrene phase, was observed in DSC experiments. The glass transition of the ionic domains was detected by dynamic mechanical measurements as a shoulder on the low-temperature side of the polystyrene. This unexpectedly low value for the Tgof the ionic domains was attributed to plasticization by water. The presence of water in these ionomers was related to the method of preparation of the materials.
Introduction The field of ionomers has received increasing attention in the past few decades, from academic as well as industrial researchers.'p2 The unique properties generally displayed by ionomers have been related to the presence of ionic aggregates within the organic polymer matrix. Numerous investigations have been conducted in an attempt to characterize these aggregates and determine the factors influencing their formation. While the exact structure of the aggregates has not yet been fully elucidated, evidence for the occurrence of aggregation, whatever the structure of the aggregates, is plentiful. The increase in glass transition temperature of most ionomers with ion content may be directly related to the presence of aggregates which are expected to diminish the mobility of the segments in a manner similar to that of covalent cross-links. The very large increase in the melt viscosity of ionomers with ion content may be attributed to ionic interactions between the chains which persist even at temperatures far above the glass transition temperature of these systems. Selective plasticization of the ionic groups with polar plasticizers has been very effective in lowering the melt viscosity of certain ionomers, e.g., sulfonated styrene i o n o m e r ~confirming ,~ the above assumption. The rate of stress relaxation has also been found to decrease with the ion content, and, above a certain proportion of ions, the breakdown in time-temperature superposition suggests the occurrence of a second relaxation mechanism associated with the presence of larger aggregates. The dynamic mechanical behavior of most ionomers also displays features that may be attributed to the presence of ionic aggregates: there is usually an additional, high-temperature peak in the tan 6 curves and a decrease in the slope of the storage modulus curves with ion content. The styrene-vinylpyridinium systems described in the preceding article of this issue are notable exceptions in this respect. All of the fundamental studies on the aggregation phenomenon in ionomers have been conducted on random copolymers. The introduction of ions in these systems has generally been achieved by one of two methods. The technique that is probably the most widely used is copolymerization with an ionizable comonomer; this is usually done by free-radical polymerization of two comonomers possessing reactivity ratios permitting the more or less random distribution of the ionizable groups along the chain. After isolation, these copolymers are transformed into ionomers using appropriate reactions. The second technique is by a postpolymerization reaction on the homopolymer. This method is especially useful in the Present address: Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 9B4.
0024-9297/87/2220-0760$01.50/0
preparation of a family of ionomers possessing identical backbones but different pendant ionic groups. Sulfonated and carboxylated styrene i ~ n o m e r sfor , ~ example, have been synthesized from polystyrene; a family of polyp e n t e n a m e r ~ has ~ - ~also been produced in this way. The ionomers produced by the techniques described above, however, will present a random distribution of ionic groups along the polymeric chain. A different type of ionomers, halatotelechelic polymers, has been produced and studied extensively in Teyssie's laboratory. Their study centered primarily on telechelic polybutadiene and polyisoprene prepolymers of relatively low molecular weight, capped with carboxylic acids. Quantitative neutralization with alkaline earth methoxides produced higher molecular polymeric materials, halatotelechelics, through linkage of the chains by the divalent cations. The ion content was varied by the use of telechelic prepolymers of different chain lengths. Thermal?Jo dilute ~olution,'~-'~ morph~logical,~~ and vis~oelastic,~J~J~ studies revealed that the presence of ions in these systems sometimes manifests itself in a different fashion than in random ionomers. Telechelic three-arm-star polyisobutylene polymers terminated with a sulfonic acid group have been prepared and studied by Kennedy, Wilkes, and c o - w ~ r k e r s . ~The ~J~ sulfonated polymers were neutralized with various metal hydroxides to produce the ionomers. While the acidic forms of these polymers are liquids at room temperature, the salts are rubbery materials with very unusual properties. Mechanical and structural s t u d i e ~ ' suggested ~,~ the occurrence of network formation in the salts. Melt viscosity21and stress-strain22 measurements demonstrated the marked improvement in the properties of these materials upon neutralization. Solution viscosity results23 were in agreement with the findings of Teyssie's group."J2 In an attempt to characterize the effect of ion placement on the state of aggregation in ionomers, styrene-4-vinylpyridinium ionomers of different architectures were prepared and studied in this laboratory. Two architectures were investigated, random and ABA (A, vinylpyridinium; B, styrene) block copolymers. The styrene-vinylpyridinium system was chosen because of the possibility of producing well-characterized materials of both architectures and also because relatively little work had been done to date on random ionomers bearing a positive charge. The study of the random copolymers has been described in the first publication of this series.24 The investigation of the ABA block ionomers is the subject of the present article. It is expected that the block ionomers will present properties that are very different from those of the random ionomers, on the one hand, and from those of the telechelic ionomers, on the other. Nonionic block copolymers have been known for a long time and have been studied extensively. Their most in0 1987 American Chemical Society
Macromolecules, Vol. 20, No. 4, 1987 teresting properties are a consequence of the general incompatibility of the two, or more, types of polymeric segments present in the copolymer. Because the segments are chemically linked, however, phase separation may only occur on a relatively small scale, and these systems are usually described as microphase separated. The morphology of these materials is influenced by factors such as the chemical nature of the segments, their respective lengths, the architecture of the copolymer (AB, ABA or (AB),), and the solvent from which the material was cast. The effect of microphase separation on the thermal and dynamic mechanical properties of block copolymers is discussed briefly in the Results and Discussion. Investigations of ionic block copolymers, i.e., block copolymers comprising at least one segment made of ionic repeating units, are relatively few. Extensive work on the solution properties of styrene-4-vinylpyridinium AB block copolymers has been done by Gallot's group. The emulsifying properties of graft and block copolymers containing a hydrophobic segment and a hydrophilic one, poly(2vinylpyridinium chloride), were The investigation of the micellization behavior of such systems has centered mainly on the study of styrene-vinylpyridine block copolymers fully quaternized with ethyl bromide, although graft copolymers possessing a styrene backbone and vinylpyridinium grafts have also been examined.29 A summary of the results and conclusions reached by these investigators on the behavior of these two systems has been published.30 A more detailed description of the investigation of the micellization behavior of the block copolymers as a function of the quality of the solvent?l the molecular characteristics of the copolymers?* the temperature, and the salt c ~ n c e n t r a t i o has n ~ ~been published in a series of three articles. A review on ionic block copolymers has been published recently by Gallot.% It should be noted that the majority of the systems described in this article were characterized by a large ionic segment length and, therefore, were not true ionomers. Aside from this thorough investigation, very little work relevant to this publication has been conducted on block copolymers containing vinylpyridinium segments. A study of some optical and mechanical properties of a styrenebutadiene-2-vinylpyridine block copolymer quaternized with hydrochloric acid35showed a marked increase (of about 1 order of magnitude) in the height of the rubbery plateau upon quaternization, which was attributed to an "ionomeric behavior". Aside from the latter study, vinylpyridinium block copolymers do not seem to have ever been examined as ionomers; most of the studies have been oriented toward applications of these systems. Finally, a preliminary study on AB block ionomers containing carboxylic acid salts has been conducted in McGrath's l a b ~ r a t o r y . ~ ~Copolymers ,~' consisting of a styrene block and an alkyl methacrylate block were synthesized by anionic polymerization. Subsequent hydrolysis produces the ionomers; different ion contents were obtained by varying the extent of hydrolysis. Thermomechanical analysis of one of the block ionomers showed that the rubbery plateau was much higher than that of the nonionic material. Styrene-methoxypoly(ethy1ene glycol methacrylate) ABA ionomers have been prepared recently by Khan and c o - ~ o r k e rand s ~ ~displayed physical properties superior to those of the corresponding homopolymers. The ABA block copolymers studied in our investigation were also prepared by living anionic polymerization of styrene and 4-vinylpyridine. The 4-vinylpyridine end blocks were quaternized with methyl iodide to produce the
Styrene-Vinylpyridine Block Copolymers 761
sample 1 2 3 4 a
Table I Sample Compositions" 4VP content, no. of 4VP mol % units/end block 2.4 12 4.7 24 7.0 35 9.1 46
Molecular weight of styrene block is lo5.
ionomers. As it seemed desirable to study materials possessing fully ionized end blocks, ie., materials for which every unit of the end blocks is ionic, polymers of different ion contents were prepared by producing copolymers of different 4-vinylpyridine end block lengths but having an identical styrene middle block size. The synthesis of these materials was conducted on an elaborate polymerization line constructed in this laboratory. A brief description of the line as well as of the procedure followed to prepare the block copolymers has been presented before39and will be the subject of a more extensive future publication. The present article describes the thermal and dynamic mechanical behavior of the styrene-4-vinylpyridinium ABA block ionomers. Experimental Section Preparation of the Block Copolymers. The block copolymers were synthesized by living anionic polymerization performed on a polymerization line constructed in this laborat0ry.3~The polymerization of styrene was initiated with sodium naphthalene to produce chains possessing two living ends; enough styrene was added to yield styrene chains of a molecular weight of about lo5. Each living end was capped with one molecule of 1,l-diphenylethyleneto reduce the reactivity of the anions and prevent side reactions with the pyridine moiety of the vinylpyridine monomer.*O Enough 4-vinylpyridine (4VP) was then added (as a 1% solution of 4VP in dry THF39)to obtain a sample containing about 2.5 mol % of 4VP or end blocks of about 12 units of 4VP. A certain amount of the polymerization mixture was withdrawn, using the transferring apparatus,39and deactivated with a few drops of methanol. About 20 g of polymer containing 2.4 mol % 4VP was eventually recovered from that solution. Enough 4VP monomer was then added to the remainder of the active polymerization solution to produce a polymer containing about 5 mol % 4VP or end blocks of about 25 units of 4VP. Again a predetermined amount of the polymerization mixture was withdrawn to yield about 20 g of polymer of this composition. Two subsequent additions of 4VP, each followed by transfer of a certain volume of polymerization solution, produced polymers containing about 7.5 and 10 mol % 4VP or about 32 and 50 units of 4VP per end block, respectively. A series of copolymers possessing absolutely identical styrene middle blocks (Le., styrene blocks of identical molecular weight and polydispersity) and various 4VP end block lengths was thus produced. The 4VP contents of the materials were determined by FTIRZ4and are listed in Table I. Quaternization. Attempt8 to quaternize the styrene-4vinylpyridine ABA block copolymers following the procedure employed for the random materials (by refluxing with excess methyl iodide in dry THFN)proved to be unsuccessful,as gelation occurred minutes after reflux had started. Although it had been shown that gelation does not prevent quantitative quaternization$ it was decided to develop an alternate method that would yield quaternized samples of a more practical texture. The alternative involved soaking of a molded sample of the nonionic block copolymer into a 80/20 (v/v) absolute ethanol/methyl iodide solution for 3 days, at the reflux temperature. (a) Materials. Absolute ethanol was used without further purification. Methyl iodide was distilled under nitrogen at 42-43 " C , a few hours before use. (b) Procedure. The desired amount material (0.8-1.0 g) was compression molded into a rectangular slab at a temperature of about 140 O C and removed from the mold at about 90 OC. The
762 Gauthier and Eisenberg
molded sample was then dried in a vacuum oven at 70 OC overnight. All the glassware used was dried in an oven for a few hours, at about 120 "C. A condenser was connected to a 125-mLErlenmeyer flask equipped with a standard tapered neck, while still hot. The assemblywas kept under a positive pressure of nitrogen. Absolute ethanol (40 mL) and methyl iodide (10 mL) were added, and the molded sample was dropped in. The mixture was then refluxed under nitrogen for 3 days, producing a yellowish sample. After cooling, the sample was removed, soaked in fresh absolute ethanol for a few minutes, and transferred to a heating pistol. The pistol was evacuated overnight at room temperature and then heated to about 80 "C for 3 days to effect complete removal of the solvent and reactant. (c) Analysis. A small piece of quaternized material was broken off after the final molding of each sample and remolded into a thin film in order to determine the degree of quaternization by FTIR analysis.24Quantitative quaternization was achieved for all samples. Thermal Properties. The thermal properties of the ionic and nonionic block copolymers were determined on a Perkin-Elmer DSC 11. The samples were dried under vacuum at 80 "C for 3 days prior t o measurements. The samples were heated on the DSC head at a temperature of 150 O C for about 5 min, quenched to 50 O C , and scanned at a rate of 20 'C/min. Duplicate determinations were made for each sample. Dynamic Mechanical Properties. The dynamic mechanical measurements were made on a torsion pendulum. (a) Sample Preparation. Rectangular slabs of the nonionic materials were obtained by compression molding at about 140 OC.
The quaternized samples were also compression molded by slipping the quaternized slabs into a slightly bigger mold and heating at about 130 "C. The samples were dried for 3 days at 80 O C under vacuum prior to the torsion pendulum measurements. The unexpected dynamic mechanical behavior (described in detail in the Results and Discussion) of these samples, however, suggested the desirability of developing a second method for the preparation of the quaternized materials, in order to break the ionic interactions prior to remolding. This was achieved by dissolving the quaternized samples and reprecipitatingthem in a nonsolvent. Different solvents and mixed solvents were tested. It was found that the common styrene ionomer solvents such as toluene/ methanol, THF/methanol, or Me2S0 did not dissolve these materials; however, both dimethylformamide and a Me2SO/ methanol mixtures proved to be effective. Because of the greater reproducibility in experimental conditions expected from a pure solvent, dimethylformamide (DMF) was chosen. Two slabs of quaternized material (1.5-2.0 g total) were dissolved in 30 mL of DMF to make a 5% solution; the polymer was precipitated in acetone. It was dried under vacuum at room temperature overnight and then further at about 80 "C for 3 days. It was compression molded at the same temperature as the undissolved, quaternized samples. The remolded samples were dried at 80 O C under vacuum for 3 days before the dynamic mechanical experiments. (b) Plasticization with Me2S0. About 0.8 g of material containing 9.1 mol % of 4-vinylpyridine was quaternized and molded as described before (firstmethod, without reprecipitation). It was dried for 3 days at 80 O C under vacuum in a heating pistol. The pistol was then allowed to cool and filled with dry nitrogen, and the sample was quickly transferred to an Erlenmeyer containing 100 mL of Me2S0. The flask was stoppered and the sample left to soak for a few days. It was then removed, the surface dried, and the sample placed under vacuum, at room temperature, for about 1 h.
Results and Discussion Block copolymers generally display physical properties that are very different from their random counterparts; while random systems exhibit properties that are a weighed average of the properties of the two components, block copolymers will usually retain the characteristics of each component. The reason for this difference in behavior is
Macromolecules, Vol. 20, No. 4, 1987 related to the occurrence of phase separation in most block copolymer systems. Because of the frequent incompatibility of the two types of segments present, block copolymers generally exhibit two distinct glass transitions a t temperatures very close to those of the corresponding homopolymers. This is in marked contrast to the behavior of random copolymers, which display a single Tg, at a temperature intermediate between the T, of the two homopolymers and dependent upon the composition of the material. The dynamic mechanical properties of block copolymers also reflect the microphase separation encountered in these systems. As expected, the variation of the loss tangent (tan 6) with temperature exhibits two peaks corresponding to the two distinct glass transitions of the sample, the magnitude of each peak being related to the amount of each component present in the material. If phase separation is incomplete, as in the case of a sample cast from a mutual solvent, a peak a t an intermediate temperature may also be present, reflecting the presence of a certain degree of mixing of the two types of segments. The variation of the storage modulus with temperature also displays unique features, drastically different from the behavior of random copolymers and, in fact, much closer to that of immiscible polymer blends, because of the occurrence of phase separation in both blends and blocks. A variation in the composition of a random copolymer does not alter the shape of the storage modulus curve but simply shifts the whole curve on the temperature axis, reflecting the change in the glass transition temperature of the material with composition. A change in the composition of a block copolymer, however, is reflected by a shift in the height of the rubbery plateau. Two inflection points are also observed, as expected, reflecting the two distinct glass transitions of the materials. Thermal Properties of Styrene-4-Vinylpyridine Block Copolymers. (a) Nonionic Blocks. As mentioned above, block copolymers generally display two glass transition temperatures characteristic of each type of segment. DSC traces of the unquaternized blocks of different 4VP contents show only one transition, however, which is associated with the glass transition of the polystyrene segments. This is not surprising in view of the relatively small amount of 4VP present in these samples, which may be below the detection limit of the instrument. The absence of a transition around the glass transition temperature of poly(4-vinylpyridine)may also be related t o their architecture. Because these polymers are of the ABA type, the length of each 4VP end block is only half of the total percentage of 4VP in the copolymer. For the sample containing 9 mol % ' J 4VP, for example, each end block has a molecular weight of only about 4500 (based on a middle block length of lo5) which will be characterized by a glass transition temperature lower than that of poly(4-vinylpyridine) and probably close to the T g of polystyrene. Thus, if the variation of Tgwith molecular weight for poly(4-vinylpyridine) is approximately the same as for polystyrene, a poly(4VP) chain of molecular weight 4500 should have a Tgof about 115 (b) Ionic Blocks. Because the Tgof the poly(viny1pyridinium) segments should be well above 200 "C and the maximum temperature to which the samples may be heated is around 150 "C due to the occurrence of dequaternization above this t e m p e r a t ~ r ethe , ~ ~ionic blocks studied here were expected to show only one transition a t around 106 OC, related to the T , of the polystyrene domains. The DSC traces confirm this hypothesis. A slight increase in the temperature of this transition with ion
Macromolecules, Vol. 20, No. 4, 1987
Styrene-Vinylpyridine Block Copolymers 763
Table I1 Variation of T,with Ion Content for Ionic Block and Random Copolymers glass transition temp, O C block random' nonionic ionic ionic 104 104 104 105 105 110 106 106 118 105 108 124 105 108 131
ion content, mol % 0.0 2.4 4.7 6.7 9.1
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